The present invention relates to a shift control apparatus for an automatic transmission and, especially, to a shift control apparatus for executing such a proper shift as is done by a changeover of friction elements, of which one friction element is brought into disengagement whereas another friction element is brought into engagement.
In the automatic transmission, there is a shift to be executed by the so-called xe2x80x9cchangeover of friction elementsxe2x80x9d, in which one friction element is brought into disengagement as the working oil pressure lowers whereas another friction element is brought into engagement as the working oil pressure rises.
Herein: the friction elements to be changed over from an engaging state to a disengaging state at the time of said changeover shift will be called the xe2x80x9cdisengagement side friction elementsxe2x80x9d; the working oil pressure will be called the xe2x80x9cdisengagement side working oil pressurexe2x80x9d; and the friction elements to be switched from the disengaging state to the engaging state will be called the xe2x80x9cengagement side friction elementsxe2x80x9d; and the working oil pressure will be called the xe2x80x9cengagement side working oil pressurexe2x80x9d.
At said changeover shift, the control to lower the disengagement side working oil pressure for bringing the disengagement side friction element into disengagement and the control to raise the engagement side working oil pressure for bringing the engagement side friction element into engagement are known in the prior art, as disclosed in Unexamined Published Japanese Patent Application No. 1-224549, for example.
By raising an engagement side working oil pressure PC from a shift command instant t1, as illustrated in FIG. 13(a), an engagement side friction element is stroked against a return spring, and it is detected by an oil pressure switch to be turned ON at PC=P1 that the engagement side friction element ends a loss stroke.
A disengagement side working oil pressure P0 is so abruptly lowered to P4 for the time period from a shift command instant t1 to an instant of the loss stroke end detection of the engagement side friction element as to take an engaging capacity just before the disengagement side friction element starts to slip, and is so slowly lowered till an instant t5 as to effect the aforementioned changeover, until it is abruptly lowered to 0.
On the other hand, the engagement side working oil pressure PC is so abruptly raised to an initial pressure P2 that the aforementioned changeover may be started at and after the detection instant t2 of the loss stroke end of the engagement side friction element to start a torque phase, and is further raised at a predetermined slow shelf pressure gradient to a pressure P3, at which an inertia phase ends, until it is raised to the highest level till an instant t7.
In a common sense, during the shelf pressure control of the engagement side working oil pressure PC, the engagement side friction element has to share the transmission input torque. It is, therefore, ordinary that the shelf pressure of the engagement side working oil pressure PC is raised, as indicated by a broken line, as an engine throttle opening degree TVO increases.
When the throttle opening degree TVO is high, therefore, the engagement side working oil pressure PC is abruptly raised for advancing the torque phase from the instant t3 to a later instant t4.
Here in the shift control apparatus of the prior art thus far described, the changing rate at the time when the engagement side working oil pressure PC is raised to the initial shelf pressure from the detection instant t2 of the loss stroke end of the engagement side friction element is given a constant gradient independently of the throttle opening degree TVO. Therefore, this gradient has to be relatively steepened for ending the torque phase within a predetermined short time period even for a high throttle opening degree, and the following problem may occur.
Here, the time-series changes of a turbine speed Nt (or a transmission input speed) and a transmission output torque TO at a high throttle opening degree for determining the engagement side working oil pressure PC, as indicated by a broken line, are illustrated in FIG. 13B. On the other hand, the time-series changes of a turbine speed Nt (or the transmission input speed) and the transmission output torque TO at a low throttle opening degree for determining the engagement side working oil pressure PC, as indicated by a solid line, are illustrated in FIG. 13C.
At the high throttle opening degree, the transmission output torque TO is relatively high, as illustrated in FIG. 13B, even if the torque falls at TOT in the torque phase, the transmission output torque TO does not take a negative value, and no serious tossing shock occurs just after the fall.
However, the changing rate at which the engagement side working oil pressure PC is raised to the initial shelf pressure from the loss stroke ending instant t2 of the engagement side friction element has a constant gradient independently of the throttle opening degree TVO. As apparent from the comparison of the turbine speed Nt illustrated in FIGS. 13B and 13C, therefore, the shift advances at a rate similar to that of the high throttle opening degree even at the low throttle opening degree having the small transmission output torque TO so that the turbine speed Nt reaches that (i.e., the speed equal to a transmission output speed No because Nt e of the shift to the 3rd speed having a gear ratio 1 in the case of FIG. 13) after the shift. At the low throttle opening degree, therefore, the rising rate (or the torque phase advancing rate) of the engagement side working oil pressure PC during that period is excessive high, and the transmission output torque TO is relatively low, as illustrated in FIG. 13C. Hence, there is caused the so-called xe2x80x9czero cross of the transmission output torquexe2x80x9d, in which the fall TOT in the torque phase makes the transmission output torque TO once negative and then returns it to a positive value. There arises a problem that a serious tossing shock TOS is caused just after the zero cross.
Here, the zero cross of the transmission output torque reverses the direction of the torque for a short time thereby to generate a backlash noise between the gears in the gear transmission mechanism of the automatic transmission to cause the noise, as indicated as the sound pressure level in FIG. 13C.
This problem of the backlash or the aforementioned serious tossing shock TOS is easily transmitted to the passenger at the low throttle opening degree because the transmission output torque TO is intrinsically low, and it has to be avoided.
In the aforementioned control of the prior art, on the other hand, the instant t3 at which the disengagement side working oil pressure PO is to be abruptly lowered is usually controlled by an orifice control valve disposed in the hydraulic circuit, and this orifice control valve acts when the engagement side working oil pressure PC exceeds its set pressure. Thus, there arises a second problem that the disengagement side working oil pressure PO abruptly drops.
The set pressure of the orifice control valve is constant. Therefore, the set pressure has to be set at a high level so as to prevent the racing of the engine, i.e., an abrupt rise in the engine speed, which might otherwise be caused when the engaging capacities of the individual friction elements on the engagement side and the disengagement side become short for a shifting action at a high throttle opening degree TVO.
Where the orifice control valve is given a high set value, the engaging capacity of the disengagement side friction element is excessive at the shifting action for an intermediate or low throttle opening degree TVO. As a result, the transmission output torque TO drops (or falls) and abruptly rises (or tosses) in the deep and long torque of the torque phase. As a result, the drivability or riding comfortableness of the vehicle is adversely affected by making a smooth shifting action difficult and by causing a serious shock during the shifting action.
On the other hand, the time period for which the individual friction elements on the engagement side and on the disengagement side are in slipping states is elongated to increase the calorific power in each friction element thereby to affect the durability of the friction element adversely.
A first invention, as described in claim 1, has an object to solve the aforementioned first problem in the apparatus of the prior art, on the basis of a fact recognition that the first embodiment is caused because the changing rate at the time when the engagement side working oil pressure is raised to the initial shelf pressure from the loss stroke ending time of the engagement side friction element is made a constant gradient independently of the engine load, by changing the rising rate of the engagement side working oil pressure after the end of the loss stroke by the engagement side friction element in accordance with the engine load.
A second invention, as described in claim 2, has an object to make the aforementioned action and effect more reliable by controlling the lowering gradient of the disengagement side working oil pressure at and after the loss stroke ending time of the engagement side friction element, too.
A third invention, as described in claim 3, has an object to advance the inertia phase after the torque phase properly by properly determining the upper limit at the time when the engagement side working oil pressure is raised from the loss stroke ending time of the engagement side friction element.
A fourth invention, as described in claim 4, has an object to solve the aforementioned second problem in the prior art apparatus, by setting the working oil pressure of the disengagement side friction element constant in the changeover shifting action for a predetermined time period from the loss stroke ending time and then by lowering the working oil pressure at a predetermined gradient.
A fifth invention, as described in claim 5, has an object to ensure the aforementioned actions and effects by making constant the predetermined time period for keeping the working oil pressure of the disengagement side friction element in the changeover shifting action, variable according to the throttle opening degree.
A sixth invention, as described in claim 6, has an object to ensure the aforementioned actions and effects by making the constant value for holding the working oil pressure of the disengagement side friction element for the predetermined time period from the loss stroke ending time in the changeover shifting action, variable according to the throttle opening degree.
A seventh invention, as described in claim 7, has an object to ensure the aforementioned actions and effects by changing the rising gradient of the working oil pressure of the engagement side friction element and the lowering gradient of the working oil pressure of the disengagement side friction element in the changeover shifting action, in accordance with the throttle opening degree.
In order to achieve the above-specified objects, according to the first invention, in an automatic transmission having a shift to be effected by such a changeover of friction elements that, while one friction element is being brought into disengagement by lowering a working oil pressure, another friction element is brought into engagement by raising the working oil pressure, and that, after the end of a loss stroke of said another friction element was detected, the working oil pressure for said one friction element is lowered at a set gradient whereas the working oil pressure for said another friction element is raised at a predetermined gradient, it is characterized that the predetermined gradient of the working oil pressure for said another friction element is made lower for a low engine load than for a high engine load.
At the time of the changeover shift of the automatic transmission to be effected by raising the working oil pressure, while lowering the working oil pressure to bring one friction element into disengagement, to bring another friction element into engagement, after it is detected from the rise in the working oil pressure that said another friction element has ended the loss stroke, the changeover shift is advanced by lowering the working oil pressure for said one friction element at a set gradient and by raising the working oil pressure for said another friction element at the predetermined gradient.
Here in the first invention, the predetermined gradient of the working oil pressure for said another friction element is made lower at the low engine load than at the high engine load. Therefore, the changing rate at the time when the working oil pressure for said another friction element or the engagement side friction element, i.e., the engagement side working oil pressure is raised after the loss stroke end of the engagement side friction element is lower than that at the high engine load so that the shift (or the torque phase) at the low engine load for the low transmission output torque can be advanced more slowly than at the high engine load.
Therefore, the rising rate (or the torque phase advancing rate) of the engagement side working oil pressure for the low engine load after the loss stroke end is not excessively high. Even if the transmission output torque is relatively low for the low engine load the fall of the torque in the torque phase causes the zero cross of the transmission output torque, therefore, the transmission output torque in the zero cross gently changes to cause no backlash noise and no serious tossing shock just after the torque phase end.
In the first invention, on the other hand, the second invention is characterized in that the set gradient of the working oil pressure for said one friction element is made lower for the low engine load than for the high engine load.
In this second invention, the set gradient of the working oil pressure for said one friction element is made smaller at the low engine load than at the high engine load. As a result, the lowering gradient of the working oil pressure (or the disengagement side working oil pressure) for said one friction element (or the disengagement side friction element) at and after the loss stroke end of the engagement side friction element is made lower at the low engine load than at the high engine load. The drop of the disengagement side working oil pressure for the rise of the engagement side working oil pressure controlled as in the first invention can be properly timed to smoothen the changeover between the engagement side friction element and the disengagement side friction element thereby to make the action and effect of the first invention more reliable.
In the first invention or the second invention, the third invention is characterized in that the upper limit for raising the working oil pressure for said another friction element at said predetermined gradient is the sum of: a return spring pressure for balancing an oil pressure necessary at the end of a piston stroke; a torque sharing pressure for an engaging capacity barely capable of transmitting a transmission input torque; and an inertia induced pressure necessary for proceeding an inertia phase.
In the third invention, the upper limit for raising the working oil pressure for said another friction element at said predetermined gradient is the sum of: a return spring pressure for balancing an oil pressure necessary at the end of a piston stroke; a torque sharing pressure for an engaging capacity barely capable of transmitting a transmission input torque; and an inertia induced pressure necessary for proceeding an inertia phase. Therefore, the working oil pressure of said another friction element (or the engagement side friction element) for advancing the inertia phase after the torque phase takes a proper value for establishing a proper engaging capacity so that the inertia phase can be properly advanced.
According to the fifth invention, in an automatic transmission having a shift to be effected by such a changeover of first and second ones of a plurality of friction elements that said first friction element is brought into engagement by raising a working oil pressure and that after a set time in response to a pressure signal on the working oil pressure of said first friction element, said second friction element is brought into disengagement by lowering the working oil pressure, and that, after the end of a loss stroke of said first friction element was detected, the working oil pressure for said second friction element is lowered at a set gradient whereas the working oil pressure for said first friction element is raised at a predetermined gradient, it is characterized that the working oil pressure for said second friction element is held at a constant value for a predetermined time period after the end of said loss stroke and is lowered at said set gradient after lapse of said predetermined time period.
In this fourth invention, when the working oil pressure for the second friction element at and after the detection instant of the loss stroke end is to be lowered at the set gradient, the working oil pressure is kept constant for a predetermined time period from the detection instant of the loss stroke end and is then lowered at said set gradient.
As a result, the racing of the engine can be prevented for the high throttle opening degree, and a satisfactory shifting action having a torque fall of small length and depth can be realized for the intermediate or low throttle opening degree.
In the fourth invention, the fifth invention is characterized in that said predetermined time period is so changed according to a throttle opening degree that it may be the longer as said throttle opening degree is the higher and the shorter as said throttle opening degree is the lower.
In this fifth invention, said predetermined time period for keeping the working oil pressure for said second friction element constant from the detection instant of the loss stroke end is so changed according to a throttle opening degree that it may be the longer as said throttle opening degree is the higher and the shorter as said throttle opening degree is the lower. By thus setting said predetermined time period, the aforementioned action and effect of the first invention can be made reliable.
In the fourth or fifth invention, the sixth invention is characterized in that said constant value of the working oil pressure for said second friction element is so changed according to the throttle opening degree that it may be the higher as said throttle opening degree is the higher and the lower as said throttle opening degree is the lower.
In this sixth invention, said constant value for holding the working oil pressure for said predetermined time period from the detection instant of the loss stroke end for said second friction element is so changed according to the throttle opening degree that it may be the higher as said throttle opening degree is the higher and the lower as said throttle opening degree is the lower. By thus setting said constant value, the aforementioned actions and effects of the fourth and fifth inventions can be made more reliable.
In any of the fourth to sixth inventions, the seventh invention is characterized in that the rising gradient of the working oil pressure for said first friction element and the lowering gradient of the working oil pressure for said second friction element are so changed according to the throttle opening degree that said individual gradients may be the larger as said throttle opening degree is the higher and the smaller as said throttle opening degree is the lower.
In this seventh invention, the rising gradient of the working oil pressure for said first friction element and the lowering gradient of the working oil pressure for said second friction element in the changeover shifting action are made the larger as said throttle opening degree is the higher and the smaller as said throttle opening degree is the lower. Therefore, the actions and effects of the aforementioned individual inventions can be made more reliable.